Diverging water use between two species in the Caatinga dry forest leads to unequal contributions to stand-level evapotranspiration

Background/Question/Methods

Succulent trees are widely distributed across the tropics, particularly in seasonally dry tropical forests (SDTF). These low wood density trees are thought to rapidly exploit resources when availability is high. In contrast, deciduous trees with high wood density are thought to be drought-tolerant and therefore, have a wider time frame for transpiration and carbon fixation. The objective of this work was to better understand the transpiration patterns for two species with contrasting wood densities in a SDTF in Brazil. We used Granier-style thermal dissipation sensors to measure the sap flux density for five individuals per species across one wet-dry season. The low wood density species was Commiphora leptophloeos; and the high wood density species was Cenostigma pyramidale. We also measured environmental variables like rainfall, soil water content, vapor pressure deficit (VPD), and net radiation.

Results/Conclusions

Our preliminary findings suggest that the succulent low wood density species is more sensitive to decreasing water content but closely follows VPD when soil water is abundant; whereas the deciduous high wood density species has a more stable and lagged response to declines in soil water content or increases in VPD. Species-level differences in sensitivity to VPD and to drying soils, across daily to monthly timescales, could be important to understanding both abiotic controls on transpiration, and how diverging water use might influence stand-level evapotranspiration patterns.